1. The Field of the Invention
The present invention concerns a system for measuring steam flowing through a convergent-divergent nozzle at a critical condition and using these pertinent measurements to determine both steam quality and flow rate.
2. The Prior Art
Steam flooding has become an accepted practice for recovery of petroleum products from marginal fields or reservoirs that require a degree of stimulation to produce a satisfactory flow of crude petroleum. There is a need for a simple method and apparatus to determine the quality of steam at the wellhead of an injection well. Such a measurement, if simplified, would be particularly useful in determining the amount of heat which is applied to the underground reservoir by the injected steam.
The measurement or monitoring of steam quality is important since the steam's quality and thereby its reservoir or formation heatup effect directly affects the resulting production operations. Further, the quality of the steam which can be most economically injected into a particular substrate or reservoir is contingent on a number of circumstances. The latter include the age of the reservoir and the anticipated prospects for extracting commercially justified amounts of hydrocarbon products therefrom.
In brief, it is desirable that the quality of steam which is injected into each injection well be altered or adjusted to a level of quality that best conforms to the condition of the formation penetrated by that well. Clearly the quality of the steam must be known before any alteration or adjustment can be made.
It is known that in order to be particularly effective in this type of stimulation operation, the flow of injected steam must be monitored by use of metering means positioned in the steam-carrying line adjacent the wellhead. It can be appreciated that steam will normally leave the steam generator or source at a known quality, pressure and mass flow rate. As the pressurized steam flow progresses towards an injection well, however, the quality will usually be substantially decreased. A decrease in the quality can be based on such factors as the distance between the well and the source and the effectiveness of pipe insulation. It will further depend on the pipe layout including number and orientation of fittings through which the steam has to travel prior to reaching the injection port or well because of phase separation that can occur in these fittings.
It is important, therefore, as a matter of economic practicality that a flow monitoring and controlling means be instituted into the steam-carrying conduit immediately upstream of each injection wellhead. A choke mechanism in the steam line will function to constrict the steam flow to thereby allow regulation of the steam mass flow rate which enters that particular well.
U.S. Pat. No. 4,836,032 discloses the use of an orifice plate in series with a critical flow choke to provide a method of measurement for both steam quality and mass flow rate. Either the orifice plate or the choke alone can be used to measure steam quality and mass flow rate. However, a mathematical expression for steam quality through both devices is obtained by solving an independent mass flow rate equation for each device, an equation for wet steam through the critical flow choke and an equation for wet steam through a sharp-edged orifice plate.